Notice that we are dealing with arhombus (parallelogram with four equal sides, and therefore its diagonals intersect forming right angles.
Then we know that one of the angles in the triangle formed at the bottom of the image must be 90 degrees.
Now, we use the property of addition of the three angles of a triangle :
x + 3x + 7 + 90 = 180
Then we combine the terms in x :
4 x + 97 = 180
subtract 97 from both sides:
4 x = 180 - 97
4 x = 83
x = 83 / 4
x = 20.75 degrees
